Calculate SMT stencil area ratio, aspect ratio, paste volume, maximum foil thickness, or minimum aperture size for rectangular and round openings in mm, mils, µm, or inches.
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Stencil Area Ratio Formula
- AR = area ratio (dimensionless)
- A = opening area
- P = opening perimeter
- L = aperture length
- W = aperture width
- D = aperture diameter
- T = stencil foil thickness
For a rectangle, A = L × W and P = 2 × (L + W), so AR = L × W / [2 × (L + W) × T]. For a round opening, A = πD²/4 and P = πD, so AR = D / (4 × T).
All dimensions use the selected unit. Changing mm, mil, µm or inches converts entered dimensions together. Aspect ratio is the smallest opening dimension divided by thickness. An area ratio of 0.66 is a conventional industry baseline, and an aspect ratio of 1.5 is a typical laser-cut stencil guide. These geometric checks do not guarantee paste release or certify compliance; validate your actual stencil, paste and printing process.
Reference Tables
Use these as starting points. Actual release depends on stencil finish, paste chemistry, squeegee setup, and pad design. The five-particle opening guide uses the upper nominal powder size; it is not an absolute printability limit. The calculated opening area × thickness is theoretical aperture volume; actual deposited paste volume depends on transfer efficiency.
| Area ratio | Geometric guidance | Action |
|---|---|---|
| ≥ 0.75 | Extra geometric margin | Validate printing process |
| 0.66 to 0.74 | Conventional baseline range | Verify with print test |
| 0.55 to 0.65 | Below conventional baseline | Validate stencil, paste and process together |
| < 0.55 | More demanding release conditions | Reduce thickness or enlarge aperture |
| Paste type | Particle size (µm) | Opening guide (5-particle rule) |
|---|---|---|
| Type 3 | 25 to 45 | ~225 µm |
| Type 4 | 20 to 38 | ~190 µm |
| Type 5 | 15 to 25 | ~125 µm |
| Type 6 | 5 to 15 | ~75 µm |
Worked Example and FAQ
Example. A 0.30 mm by 0.25 mm rectangular aperture in a 0.10 mm foil.
- Opening area = 0.30 × 0.25 = 0.075 mm²
- Wall area = 2 × (0.30 + 0.25) × 0.10 = 0.110 mm²
- Area ratio = 0.075 / 0.110 = 0.68 (passes the 0.66 target)
- Aspect ratio = 0.25 / 0.10 = 2.5 (passes the 1.5 target)
What if my area ratio is too low? Reduce foil thickness or evaluate a larger opening. Other aperture shapes require their own opening and wall geometry. A nano-coated or electroformed stencil may improve release, but a 0.55 target requires process-specific validation.
Why does aspect ratio still matter? Aspect ratio adds a smallest-dimension check for long, narrow apertures, such as fine-pitch QFP leads. Consider both geometric checks along with process trials; neither alone guarantees reliable release.
Which target should I pick? Use your supplier and process target. The calculator offers 0.66 as a conventional baseline, 0.75 for extra geometric margin, and 0.55 only when your actual process has been validated for it. These selections are not certified IPC compliance checks.
